I'm very proficient in DNS, its my day job (for a State Gov't). If someone wants to spin up a free EC2 instance in Amazon AWS, I'd happily configure it to block whtaever you like. its literally free for 1 year. I've already used my credit card for it once so I'm not an option.
What kind of Mac do you have? ...there's several ways, you can get this setup just using your Mac with very little effort.
For example install Glimmer and enable proxy, add the site urls you want to block to your custom sites list on Glimmer. Add your IP of your Mac and port (default 8228) on your Wii U, under the proxy settings option.
Another way is to setup Internet connection sharing, if you don't use wireless connection and have it shared with your wired connection. Then put any unwanted domains in your HOSTS file routing back to 127.0.0.1 or 0.0.0.0. Just make sure your Wii U connects direct to your ICS on your Mac.